Due to the ever increasing electrification of motor vehicles, the demand for electric energy in motor vehicles is increasing considerably. In addition, the power consumers in motor vehicles impose increasing requirements to the quality of the power supply (voltage stability and low interference signal susceptibility). This does not only apply to “comfort consumers” (audio system, navigation system, air conditioning system, etc.), but in particular to function or safety relevant consumers (onboard computer, brake control system, fuel injection control system, etc.) of the motor vehicle. In the case of occurring load peaks, for example, in particular during the starting operation of the motor vehicle, this may have the effect that consumers which have already been activated do no longer function properly or are subjected to interference signals via the power supply because of the load applied by the starter. While, in the case of the audio system, this may be experienced as “merely” annoying, critical functional abnormalities may occur in other function or safety relevant consumers (onboard computer or the like). Such interference effects are particularly noticeable in the start/stop mode of motor vehicles, which for fuel economy reasons is increasingly favoured.
Another aspect is the increasing requirement to the electromagnetic compatibility (EMC) of the motor vehicles. Conventional passive filter need a comparatively large installation space. Moreover, their efficiency is limited.
The active compensation filter which will be described below requires a small installation space, has a minimal series resistance, and may replace conventional passive filters. It is thus very well suited for, e. g. electrical equipment in the automotive field.
U.S. Pat. No. 4,667,279 relates to noise rejection without significant dissipation of energy. The noise generated within a DC power supply is reduced by coupling a high pass filter to the output circuit of the supply. This produces a signal corresponding to the noise. This signal is applied to the primary winding of a transformer via an amplifier. The secondary winding is connected to the output circuit so as to tend to cancel any noise therein. The input of the high pass filter is connected across the output circuit, and the secondary winding of the transformer is connected in series with it so as to be traversed by load current.
This provides for optimized for operation in a constant voltage mode. An amplifier acts as a voltage amplifier and the phasing is such that the voltage appearing across the secondary winding is out of phase with the noise voltage so that cancellation can occur.
The high pass filter is connected to either side of the secondary winding. The input of the high pass filter is connected across a current sensing resistor through which the load current passes, and the secondary winding is connected across the output circuit so as to be in shunt with the load. The amplifier acts to vary the current in the secondary circuit in such phase with respect to the noise current flowing toward the load as to reduce the noise current from flowing in the load.
In order that the changes in noise cancellation voltage or current thus introduced in the output circuit may have the correct amplitude, the gain of the amplifier is set at an appropriate value. If the cancellation voltage or current do not have the proper phase, a phase compensation circuit is coupled to the amplifier.
U.S. Pat. No. 3,628,057 relates to a corrective circuit for use in an active band-pass filter that provides improved cancelling out of the power source signals that pass through the stop-band notch filter. Reference and orthogonal phase detectors are combined which each generate an error signal indicating the magnitude and polarity of the in phase (reference) and 90° out of phase (orthogonal) components of the power source signals passing through the stop-band notch filter. A signal generator connected to the power line produces plus and minus reference signals and plus and minus orthogonal signals. A reference and an orthogonal modulator are each connected to receive and use the respective reference and orthogonal error signals to amplitude modulate the signals from the signal generator to construct a feedback signal which is equal to and 180° out of phase with the power source signal passing through the stop-band notch filter.
European Patent Application 0 877 467 A1 relates to a device which reduces the ripple component of the signal fed to a load by a power converter via a feed line. The device has one inductor and an acquisition means for acquiring the signal fed to the load. It further has means a filter element for extracting the ripple component from the signal. Connected to the inductor are inductor feed means for generating an electric signal fed to the inductor. The inductor feed means generate the electric signal automatically varying this signal in relation to the value of the ripple component measured in the feed line. This reduces the ripple component in the signal fed to the load.
The inductor is of the coupled type with its primary circuit being connected to the load and its secondary circuit being connected to a second power amplifier generating the electric signal arranged to reduce the ripple component, and the common node being connected to the power converter. The acquisition means, the extraction means and the inductor feed means are connected into a feedback loop.
U.S. Pat. No. 5,668,464 refers to a parallel active filter which is incorporated into a switching power converter. The filter reduces ripple by injecting a cancellation current into the electrical load. The cancellation current is equal and opposite to an estimated ripple current. The estimated ripple current is produced by integration of an inductor voltage within the converter circuit. An adaptive tuning circuit is incorporated into the filter. The filter uses feedforward of an internal converter voltage to determine the necessary ripple compensation current.